Traditional techniques for introducing a printed image onto a recording material include letterpress printing and offset lithography. Both of these printing methods require a plate. To transfer ink in the pattern of the image, the plate is usually loaded onto a plate cylinder of a rotary press for efficiency. In letterpress printing, the image pattern is represented on the plate in the form of raised areas that accept ink and transfer it onto the recording medium by impression. The term "lithographic," as used herein, is meant to include various terms used synonymously, such as offset, offset lithographic, planographic, and others. By the term "wet lithographic," as used herein, is meant the type of lithographic printing plate where the printing is based upon the immiscibility of oil and water, wherein the oily material or ink is preferentially retained by the image area and the water or fountain solution is preferentially retained by the non-image area. When a suitably prepared surface is moistened with water and an ink is then applied, the background or non-image area retains the water and repels the ink while the image area accepts the ink and repels the water. The ink on the image area is then transferred to the surface of a material upon which the image is to be reproduced, such as paper, cloth, and 2 the like. Commonly the ink is transferred to an intermediate material called the blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced. In a dry lithographic printing system that does not utilize water, the plate is simply inked and the image transferred directly onto a recording material or transferred onto a blanket and then to the recording material.
Aluminum has been used for many years as a support for lithographic printing plates. In order to prepare the aluminum for such use, it is typically subject to both a graining process and a subsequent anodizing process. The graining process serves to improve the adhesion of the image to the plate and to enhance the water-receptive characteristics of the background areas of the printing plate. The graining and anodizing affect both the performance and the durability of the printing plate. Both mechanical and electrolytic graining processes are well known and widely used in the manufacture of lithographic printing plates. Processes for anodizing aluminum to form an anodic oxide coating and then hydrophilizing the anodized surface by techniques such as silication are also well known in the art, and need not be further described herein. The aluminum support is thus characterized by having a porous, wear-resistant hydrophilic surface, which specifically adapts it for use in lithographic printing, particularly where long press runs are required.
The plates for a lithographic press are usually produced photographically. The aluminum substrate described above is typically coated with a wide variety of photosensitive materials suitable for forming images for use in the lithographic printing process. Lithographic printing plates of this type are usually developed with an aqueous alkaline developing solution, which often additionally comprises a substantial quantity of an organic solvent.
To prepare a wet plate using a typical negative-working subtractive process, the original document is photographed to produce a photographic negative. This negative is placed on an aluminum plate having a water-receptive oxide surface coated with a photopolymer. Upon exposure to light or other radiation through the negative, the areas of the coating that received radiation (corresponding to the dark or printed areas of the original) cure to a durable oleophilic state. The plate is then subjected to a developing process that removes the uncured areas of the coating (i.e., those which did not receive radiation, corresponding to the non-image or background areas of the original), thereby exposing the hydrophilic surface of the aluminum plate.
Throughout this application, various publications, patents, and published patent applications are referred to by an identifying citation. The disclosures of the publications, patents, and published patent applications referenced in this application are hereby incorporated by reference into the present disclosure to more fully describe the state of the art to which this invention pertains.
As is evident from the above description, photographic platemaking processes tend to be time consuming and require facilities and equipment adequate to support the necessary chemistry. Efforts have been made for many years to manufacture a printing plate, which does not require aqueous alkaline or solvent development or which only uses water for development. In addition, practitioners have developed a number of electronic alternatives to plate imaging, some of which can be utilized on-press. With these systems, digitally controlled devices alter the ink-receptivity of blank plates in a pattern representative of the image to be printed. Such imaging devices include sources of electromagnetic radiation, produced by one or more laser or non-laser sources, that create physical and/or chemical changes on plate blanks; ink jet equipment that directly deposits ink-repellent or ink-accepting spots on plate blanks; and spark-discharge equipment, in which an electrode in contact with or spaced closely to a plate blank produces electrical sparks to physically alter the topology of the plate blank, thereby producing "dots" which collectively form a desired image as for example, described in U.S. Pat. No. 4,911,075. Because of the ready availability of laser equipment and its amenability to digital control, significant effort has been devoted to the development of laser-based imaging systems.
In one such system, argon-ion, frequency-doubled Nd-YAG, and other infrared lasers are used to expose photosensitive blanks for traditional chemical processing, as for example described in U.S. Pat. Nos. 3,506,779; 4,020,762; 4,868,092; 5,153,236; 5,372,915; and 5,629,354. In an alternative to this approach, a laser has been employed to selectively remove, in an imagewise pattern, an opaque coating that overlies a photosensitive plate blank. The plate is then exposed to a source of radiation, with the unremoved material acting as a mask that prevents radiation from reaching underlying portions of the plate, as for example described in U.S. Pat. No. 4,132,168. However, the need for high writing speeds, coupled with the constraint of the low-powered lasers favored by industry, has resulted in a requirement for printing plates that have a very high photosensitivity. Unfortunately, high photosensitivity almost always reduces the shelf life of these plates.
Another approach to laser imaging uses thermal-transfer materials, as for example described in U.S. Pat. Nos. 3,945,318; 3,962,513; 3,964,389; 4,395,946; and 5,395,729. With these systems, a polymer sheet transparent to the radiation emitted by the laser is coated with a transferable material. The transfer side of this construction is brought into contact with an acceptor sheet, and the transfer material is selectively irradiated through the transparent layer. Irradiation causes the transfer material to adhere preferentially to the acceptor sheet. The transfer and acceptor materials exhibit different affinities for fountain solution and/or ink, so that removal of the transparent polymer sheet with the unirradiated transfer material still on it leaves a suitably imaged, finished plate. Typically, the transfer material is oleophilic, and the acceptor material is hydrophilic. Plates produced with transfer type systems tend to exhibit short useful lifetimes due to the limited amount of material that can effectively be transferred. Airborne dirt can create an image quality problem depending on the particular construction. In addition, because the transfer process involves melting and resolidification of material, image quality further tends to be visibly poorer than that obtainable with other methods.
Other patents describe lithographic printing plates comprising a support and a hydrophilic imaging layer which, upon imagewise laser exposure, becomes oleophilic in the exposed areas while remaining hydrophilic in the unexposed areas, as for example disclosed in U.S. Pat. Nos. 3,793,033, 4,034,183; 4,081,572; and 4,693,958. However, these types of lithographic printing plates suffer from the lack of a sufficient degree of discrimination between oleophilic image areas and hydrophilic non-image areas, with the result that image quality on printing is poor.
Early examples utilizing lasers used the laser to etch away material from a plate blank to form an intaglio or letterpress pattern, as for example described in U.S. Pat. Nos. 3,506,779 and 4,347,785. This approach was later extended to production of lithographic plates, e.g., by removal of a hydrophilic surface to reveal an oleophobic underlayer, as for example described in U.S. Pat. No. 4,054,094. These early systems generally required high-power lasers, which are expensive and slow.
Other infrared laser ablation-based systems for imaging lithographic plates have been developed. These operate by laser-induced ablative removal of organic coating layers, which are coated onto a substrate such as a polyester/metal laminate or onto a polymer coating on a metal support. Use of these polyester or polymer coating materials between the ablation coating and the heat absorbing metal support provides a thermal barrier material which reduces the amount of laser energy required to ablate or fully remove the ablative-absorbing layer and any overlying surface layer, as for example described in Canadian Pat. No. 1,050,805 and in U.S. Pat. Nos. 5,339,737; and 5,353,705. The laser exposure thus removes one or more plate layers, resulting in an imagewise pattern of features on the plate. When the layers removed by laser ablation are the image regions that accept ink, the plates are negative working. When lasers with a large spot size are used for imaging a negative working plate, the size of the smallest printed dot is about as large as the spot size. Consequently, the image quality on printing may not be high. For example, a 35 micron laser spot size would print its smallest dot size at about 35 microns with a negative working plate. On a 200 lines per inch (1 pi) halftone screen, this is equivalent to a 5% to 6% dot.
U.S. Pat. No. 5,353,705 discloses a basic plate construction of a lithographic plate having a secondary ablation layer intermediate between a substrate and a surface layer, such as a hydrophilic metal substrate and a radiation-absorptive and ablatively absorbing surface layer. The secondary ablation layer performs the protective or thermal barrier function that shields the substrate from the thermal effects of imaging radiation. The secondary ablation or thermal barrier layer of the '705 patent is ablated only partially in response to ablation of the ablative-absorbing layer, is preferably substantially transparent to the laser radiation and thereby not characterized by ablative absorption of imaging radiation, and differs from the surface layer in its affinity for at least one printing fluid selected from the group consisting of ink and a fluid that repels ink, i.e., when the surface layer is ink-receptive and/or not receptive to a fountain solution, the thermal barrier layer is not ink-receptive and/or is receptive to a fountain solution, respectively. When the basic plate construction described in the '705 patent has an ink receptive surface layer, and the thermal barrier or secondary ablation layer is receptive to a fountain solution and thus is not ink receptive, a positive working, wet lithographic plate results since the portions not removed by ablation are the image regions that accept ink. Suitable polymeric materials for the secondary ablation layer of the '705 patent include, but are not limited to, polymethyl methacrylates, cellulosic ethers and esters, polyesters, and polyurethanes. Hexamethoxymethylmelamine with p-toluenesulfonic acid may be added to these polymeric materials.
U.S. Pat. No. 5,493,971 describes an example of such a positive working, wet lithographic plate. Its plate construction includes a hydrophilic metal substrate, a polymeric, hydrophilic protective or thermal barrier coating which also may serve as an adhesion-promoting primer, and an ink-accepting oleophilic surface layer characterized by ablative absorption of imaging radiation. The imaging laser interacts with the ablatable surface layer, causing ablation thereof. After laser ablation imaging which removes at least the surface layer and also at least some of the hydrophilic protective layer as shown in FIG. 2 of the '971 patent, the plate is then cleaned with a suitable solvent, e.g., water, to remove portions of the hydrophilic protective layer still remaining in the laser-exposed areas. Since the hydrophilic protective layer is partially ablated in the '971 patent, but is not characterized by ablative absorption of imaging radiation, this hydrophilic protective layer must not absorb the laser imaging radiation. It is thus similar to the secondary ablation layer of the '705 patent which is partially ablated and may be substantially transparent to the laser imaging radiation and thus not characterized by ablative absorption of the surface layer. In the '971 patent, depending on the solubility properties of the residual plug of the partially ablated hydrophilic protective layer in the cleaning solvent, e.g., water, the cleaning step reveals the hydrophilic protective coating at less than its original thickness, or reveals the hydrophilic metal substrate in the areas where the hydrophilic protective coating is entirely removed by the cleaning step. After cleaning, the plate behaves like a conventional positive working wet lithographic metal plate on the printing press.
However, adhesion of the remaining ink-accepting surface coating to the hydrophilic protective layer has proven a difficult problem to overcome. Loss of adhesion can result if the protective hydrophilic thermal barrier layer in the image or printing areas of the plate is damaged or degraded during the laser imaging and cleaning process of the '971 patent. For example, too much solvent or solubilization action by the cleaning solution or the fountain solution on press may erode the walls of the image areas, eliminating the underlying support provided by the hydrophilic barrier layer around the periphery of the image feature and degrading small image elements. This is particularly problematical when the hydrophilic protective coating layer is partially ablated and probably further removed by the cleaning step and the action of the fountain solution such that the original surface of this protective coating layer is removed. This fully exposes the interface between the ink-accepting layer and the hydrophilic protective coating layer, as well as some of the wall of the hydrophilic protective coating layer at the edge of the image feature, to these wet cleaning and fountain solutions. This may lead to a major loss of image quality. Small dots and type may be removed during the cleaning step or early in the print run. Efforts to improve the adhesion of the laser ablatable surface coating and/or its durability to permit longer printing runs typically leads to a significant increase in the laser energy required to image the plate. International Publication No. WO 99/37481 discloses novel positive working, wet lithographic printing plates and methods for preparing such lithographic printing plates, which overcome this adhesion problem.
U.S. Pat. No. 5,605,780 describes a laser-ablatable lithographic printing plate comprising an anodized aluminum support having thereon an oleophilic image-forming layer comprising an infrared-absorbing agent dispersed in a film-forming cyanoacrylate polymer binder. The hydrophilic protective layer has been eliminated. The '780 patent describes low required laser energy, good ink receptivity, good adhesion to the support, and good wear characteristics. Print runs of more than 8200 impressions are shown in the examples.
U.S. Pat. No. 5,339,737 and Reissue Pat. No. 35,512 describe a variety of ablation-type lithographic plate configurations for use with laser diode imaging apparatus. These configurations include an ablation layer, which volatilizes into gaseous and particulate debris in response to infrared imaging radiation. As used herein, the term "ablation" refers to the volatilization of a layer or a material into gaseous and particulate debris in response to imaging radiation, which ablation results in a loss of mass or weight in the layer or material. For example, U.S. Pat. No. 5,493,971 describes a complete or 100% ablative loss of the ablative layer during the laser ablation imaging process, and FIG. 3A of International Publication No. WO 99/37481 describes a partial ablative loss of about 50% or greater of the ablatable layer during the laser ablation imaging process.
Lithographic printing members are now commonly imaged by lower-power laser ablation imaging mechanisms. A major problem with these infrared laser ablation-based systems for imaging lithographic plates has been environmental. Because these operate by laser-induced destruction or removal of organic polymers and other organic or inorganic materials which are coated in one or more layers overlying a substrate, airborne debris and vapors are produced during imaging which may be hazardous to the laser equipment and to the personnel who operate the equipment. Expensive equipment is generally required to contain the debris and to capture the gases.
Despite the many efforts directed to the development of a laser imageable wet lithographic printing plate, there still remains a need for plates that require no alkaline or solvent developing solution, that perform like a conventional lithographic printing plate on press, that are sensitive to a broad spectrum of laser energy such as 700 nm to 1150 nm, that provide a high resolution and durable image, and that do not produce debris and vapor requiring expensive and complex containment equipment.